The development of dual-band dual-polarization arrays is of most interest in for instance cellular telecommunication services. Both second generation (2G) cellular services, such as the European GSM900, GSM1800 and the American AMPS and PCS1900, and third generation (3G) cellular services (such as UMTS) take advantage of polarization diversity in their network of base station possible the size of the antenna installation. Keeping a minimum size for the antenna set-up in a BTS becomes a major issue when taking into account that the growth on the service demands forces operators in increasing the number of BTS, which is starting to produce a significant visual and environmental impact on urban and rural landscapes. The problem becomes particularly significant when the operator has to provide both 2G and 3G services, because since both kinds of services operate at different frequency bands the deployment of both networks using conventional single-band antennas implies doubling the number of installed antennas and increasing the environmental impact of the installation. Therefore, the invention of dual-band dual-polarization antennas, which are able to cope simultaneously with two services at two different bands, appears as a most interesting issue.
The development of multiband antennas and antenna arrays is one of the main engineering challenges in the antenna field. There is a well-known principle in the state of the art that states the behavior of an antenna or antenna array is fully dependent on its size and geometry relative to the operating wavelength. The size of an antenna is fully dependent on the wavelength, and in an antenna array, the spacing between elements is usually fixed and keeps a certain proportion with respect to the wavelength (typically between a half and a full wavelength). Due to this very simple principle, it is very difficult to make an array to operate simultaneously at two different frequencies or wavelengths, because is difficult to make the antenna element geometry to match in size two different wavelengths and similarly, it is difficult to find an spatial arranging of the antenna elements that meets the constraints of both wavelengths at the same time.
The first descriptions of the behavior of antenna arrays were developed by Shelkunoff (S. A. Schellkunhoff, “A Mathematical Theory of Linear Arrays,” Bell System Technical Journal, 22, 80). That work was oriented to single-band antennas. Some first designs of frequency independent arrays (the log-periodic dipole arrays or LPDA) were developed in the 1960's (V. H. Rumsey, Frequency-Independent Antennas. New York Academic, 1966; R. L. Carrel, “Analysis and design of the log-periodic dipole array,” Tech. Rep. 52, Univ. Illinois Antenna Lab., Contract AF33(616)-6079, October 1961; P. E. Mayes, “Frequency Independent Antennas and Broad-Band Derivatives Thereof”, Proc. IEEE, vol. 80, no. 1, January 1992). Said LPDA arrays where based on a non-uniform spacing of dipole elements of different sizes and were designed to cover a wide range of frequencies, however due their moderate gain (10 dBi) these designs have a restricted range of application and would not be suitable for applications such as for instance cellular services, where a higher gain (above 16 dBi) is required. Also, neither the horizontal beamwidth (too narrow for BTS) nor the polarization and mechanical structure of said LPDA antennas match the requirements for BTS.
Recently some examples of multiband antenna arrays have been described in the state of the art. For instance patent PCT/ES99/00343 describes an interleaved antenna element configuration for general-purpose multiband arrays. A co-linear set-up of antenna elements is described there, where the use of multi-band antenna elements is required at those positions where antenna elements from different bands overlap. The general scope of that patent does not match the requirements of some particular applications. For instance it is difficult to achieve a dual-band behavior following the description of PCT/ES99/00343 when the frequency ratio between bands is below 1.5, as it is intended for the designs disclosed in the present invention. Also, that solution is not necessarily cost-effective when an independent electrical down-tilt is required for each band. The present invention discloses a completely different solution based on dual-polarization single-band antenna elements, which are spatially arranged to minimize the antenna size.
There are already existing examples of dual-band dual-polarization antennas in the market which handle simultaneously 2G and 3G services, however these are the so called ‘side-by-side’ solutions which simply integrate two separate antennas into a single ground-plane and radome (FIG. 1). The inconvenient of these antenna configurations are the size of the whole package (with up to 30 cm wide they are typically twice as much the size of a single antenna) and the pattern distortion due to the coupling between antennas. Some examples of this solutions can be found for instance in http://www.racal-antennas.com/ and in http://www.rymsa.com/. The present invention discloses a more compact solution which is achieved by means of a careful selection of the antenna element positions and the shape of said antenna elements which minimizes the coupling between them.
For the particular case where the spacing between f1 and f2 is very small, several broadband solutions are described in the prior art to operate simultaneously at both bands. However, such solutions are not suitable if an independent and different down-tilt is required for each band, which is something that can be easily solved according to the present invention.